1. Technical Field of the Invention
The present invention relates to a high frequency oscillator for generating signals of frequencies in the millimeter wave or microwave band, and more particularly to a high frequency oscillator which obtains an oscillation output by combining the outputs of a first oscillator and a second oscillator having the same oscillation frequency.
2. Description of the Related Art
A high frequency oscillator whose oscillation frequencies range approximately from 1 GHz to 100 GHz is useful as, for instance, the oscillation source of a high frequency line network interlocked with optical cables or the oscillation source of a measuring instrument. One of high frequency oscillators uses, as disclosed in the Japanese Patent Laid-Open Application No. 4-175001 (JP, 4-175001, A) for example, is a so-called push-push type oscillator which uses two oscillators generating a fundamental frequency signal and outputs a second harmonics signal that of the fundamental frequency signal by combining the outputs of these oscillators. Hereinafter, such an oscillator generating a second harmonics signal will be referred to as a double frequency oscillator.
FIG. 1 shows an example of configuration of a conventional double frequency oscillator. This double frequency oscillator is composed of resonator 1, first oscillating unit 2a, second oscillating unit 2b and combiner 3.
Resonator 1 uses, for instance, a dielectric resonator, LC elements disposed in an IC (integrated circuit), or a microstrip line. Each of oscillating units 2a and 2b involves an amplifier for oscillation and a feedback line, and they constitute first and second oscillators of the same oscillation frequency (fundamental frequency signal) f0 by sharing resonator 1. However, they are so arranged that the fundamental frequency signal outputs from the respective oscillators have a phase difference of 180 degree, i.e. to be reverse in phase to each other. Combiner 3, which is an in-phase combiner consisting of a differential amplifier for instance, combines the outputs of the oscillators and externally supplies combined output fout.
FIG. 2A to FIG. 2C show the respective output waveforms of the output of first oscillating unit 2a, the output of second oscillating unit 2b and combined output fout in the circuit shown in FIG. 1. In the circuit shown in FIG. 1, as the outputs of fundamental frequency signal f0 from first and second oscillating units 2a and 2b are reversed in phase, by 180 degrees to each other, as shown in FIG. 2A and FIG. 2B, the fundamental frequency signal components fout are cancelled with each other to be reduced to 0 in the combined output as shown in FIG. 2C. On the other hand, regarding the second harmonic signal (2f0) whose frequency is double that of fundamental frequency signal f0, as the outputs of first and second oscillating units 2a and 2b are identical in phase, they are combined by combiner 3 and outputted as combined output fout. Eventually, in the circuit shown in FIG. 1, out of the oscillation components from the oscillating units, the fundamental frequency signal and its odd number-order harmonics are cancelled with each other and are not outputted. Whereas even number-order harmonics from the oscillating units are combined and doubled in level, the second harmonic signal 2f0 becomes the highest level in combined output fout when it is outputted because even number-order harmonics of quadruple and higher-order harmonic signals are far lower in level than the second harmonic signal.
However, the second harmonic signal oscillator shown in FIG. 1, as it requires combiner 3 for combining the outputs of first and second oscillating units 2a and 2b, involves the difficulty of reducing the size. Furthermore, oscillation frequency (fundamental frequency signal) components from first and second oscillating units 2a and 2b including combiner 3 should be kept reverse in phase to, i.e. at a phase difference of 180 degrees from, each other, and this poses a difficulty in electrical circuit designing.